Why 28V DC?

"GeorgeB" wrote in message ...
On Sun, 18 Jan 2004 19:20:48 -0500, "Ron Natalie"
wrote:

"Jim Weir" wrote in message news
Jeez...that's something I've never considered. You are RIGHT, internal to that
alternator there IS 3-phase AC that is regulated and rectified to produce the
DC...

And now with three small transformers and a little surgery on the alternator, we
could have 3-phase 115VAC to run those military gyros...I wonder...yeah, I know
the frequency is a function of engine RPM...but still...
\
You think that AC coming out of those inverters is all that stable? It tends to wander
with the input voltage. Of course the real problem is that if the alternator crumps, the
battery will not put out three phase.

The voltage is pretty stable

I was referring to the freqeuncy (in response to Jim's comment about the engine RPM's).
You can actually here the whine of the inverter change pitch with the change of input voltage.

"Peter Gottlieb" wrote in message
. net...

"S Narayan" wrote in message
...

A 42V DC? shock could be pretty bad. The new Toyota Prius uses 273V for
the
traction motor, but that is very local to the hybrid system. Maybe a low
voltage higher frequency AC system might be a solution as someone
suggested
earlier in the thread. Conversion efficiencies are pretty good these
days
with more advanced controllers. Then individual components could step up
the
voltage and convert to higher DC voltages as needed.


I've worked on a lot of 48 Volt systems and it is more like an unpleasant
tingle than a bad shock.

Higher voltage means thinner wiring, cutting weight.

I meant if things got wet, it could be quite a tingle. Considering the
highly litigous public in this country, I can see the lawsuits if poeple got
even slightly "hurt". Agree about the copper savings. I grew up with 240VAC
power and that is something you don't fool around with. I have also had
shocks from the flyback transformer in TVs and ignition systems in cars, but
those are likely to get you physically hurt (uncontrolled muscle reaction)
than the shock itself.

And more easily filtered when rectified for DC.
--
Gene Seibel
Hangar 131 - http://pad39a.com/gene/plane.html
Because I fly, I envy no one.





"S Narayan" wrote in message ...

If memory serves, 3-phase power is more efficiently distributed, if all 3
phases have equal loads (either as a star or delta connected network), there
is no current in the return path (ground). That is, sum of all phases is 0.
So you can save one conductor for the same power transmitted. The generation
of 3 phase power is also easy and I believe it may also be more efficient in
terms of the generator design. The 400Hz transformer, compared to a 50/60Hz
one, requires less "iron" for the same flux generation (or less turns) since
the mutual inductance is proportional to frequency. Hence they are lighter.
However, they may have more losses due to eddy currents etc.

You used to be able to buy a "converter" for your alternator-equipped
car to turn it into a 110-volt DC supply. Good for running lights,
heaters, and series-wound motors such as those found in electric
drills, skilsaws and the like. No good for induction motors or
anything with a transformer (electronic stuff). I boought one of these
years ago, paid a handsome price for this "advanced" technology, then
took it apart to see what was in the box before I installed it.
It was the box, a household 110V duplex receptacle, a DPDT toggle
switch, an NE-2H bulb and 22K (might have been 27K) resistor. Total
value not more than eight or ten bucks, today's prices. The box would
be the most expensive item.
One pole of the switch switched the alternator field from the
regulator directly to the battery, and the other switched the
alternator output from the battery to the receptacle. A manual
throttle cable (not included) was used to set engine RPM to a value
that would fire the NE-2H, being about 110V with that resistor, and
you went to work. The battery would drain slowly as the field was
consuming about 5 or 6 amps, and you'd have to flip the switch once in
a while to recharge it. It didn't hurt the alternator, as it takes
amps, not volts, to burn it out. The diodes would suffer first, from
the voltage, I think. You'd have a hard time finding a tool that used
60 amps.
It was a handy doodad for anyone without power at the field. I
don't know if they're still made. It wouldn't work with today's
alternators with their built-in regulators.

Dan


Jim Weir wrote in message . ..
Jeez...that's something I've never considered. You are RIGHT, internal to that
alternator there IS 3-phase AC that is regulated and rectified to produce the
DC...

And now with three small transformers and a little surgery on the alternator, we
could have 3-phase 115VAC to run those military gyros...I wonder...yeah, I know
the frequency is a function of engine RPM...but still...

Jim


(Dan Thomas)
shared these priceless pearls of wisdom:

- That's probably why the alternators in your car and airplane both
-are three-phase. A 60-amp alternator weighs less than an old 25-amp
-generator.
-
- Dan

Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com